You are looking to use dirname. If you only want that one directory, you can use os.path.basename,
When put all together it looks like this:
os.path.basename(os.path.dirname('dir/sub_dir/other_sub_dir/file_name.txt'))
That should get you "other_sub_dir"
The following is not the ideal approach, but I originally proposed,using os.path.split, and simply get the last item. which would look like this:
os.path.split(os.path.dirname('dir/sub_dir/other_sub_dir/file_name.txt'))[-1]